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Osteichthyes lecture notes
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Chordates 2 – OsteichthyesRelationships of bony fish:
Features of Osteichthyes:
, Swim bladder/lung
Highly mobile fins
Reduced number of gill arches (3-4) with a gill cover (operculum)
Bony skeleton
Swim Bladder/Lung: many actinopterygians and sarcopterygians have an air-
filled chamber/chambers in the body. In lungfish and tetrapods, this structure is
used to acquire oxygen from the air. In most ray-finned fish, the lung is used for
buoyancy. Negatively buoyant sharks must swim constantly to stay afloat – ray-
finned fish can simply hang motionless in the water. It has long been suspected
that the lung is somehow related to the swim bladder – original hypothesis is that
the lung is modified from the swim bladder.
Polypterus: the most primitive living member of the ray-finned fish. Unlike more
advanced ray-finned fish, it does not have a swim bladder. It has lungs, and can
breathe air – this is a useful feature in a freshwater tropical fish, because warm,
stagnant water has little oxygen. The implication here is that lungs may have
originally evolved to help aquatic fish cope with low-oxygen water – not for life
on land. The lung was later modified into a buoyancy device.
Fins: although sharks have fins, they tend to be used mostly for steering and
stabilisation – the tail is the main source of propulsion. In bony fish, the fins tend
to be much more mobile. Sarcopterygian fish (and Polypterus) have a mobile fin
base allowing them to move the pectoral and pelvic fins – shoulder and hip
joints. The fin itself is also modified – a series of bony struts supporting a flexible
membrane, like a Japanese fan. This structure means that bony fish can fold and
unfold their fins, and change their shape, allowing them to be used for propulsion
in many species.
Advanced ray-finned fish can tightly fold the fins against the body to reduce
drag. This lets them reach higher speeds, and cruise more efficiently. Mobile fin
bases along with those lungs allow the bichir to crawl up onto land.
The high mobility of the pectoral and pelvic fins is taken to the extreme in
tetrapods, which use the fins for crawling, running, climbing, capturing prey,
flight, etc.
Modified respiratory system: sharks have anywhere from 5-7 gills, and
lamprey and hagfish have even more. Bony fish have 3 pairs of gills, covered but
a moveable operculum (gill cover). Although lost in amniotes, this system is
retained in larval amphibians. Jawless fish have primitive pumping mechanisms,
while many sharks are ram-breathers, swimming forward to force water over the
gills. The advanced pumping mechanism of bony fish means that they can
effectively oxygenate the blood even while sitting still, and could help explain
the success of bony fish in certain environments (like warm waters of tropical
rivers, or stagnant waters of lakes) where sharks and rays don’t do as well.
Bone: in bony fish the skeleton (including the jaws, vertebrae and fin rays) are
all supported by mineralised tissue.
Outside in: the first mineralised tissues in fish are external – head plates and
scales. These are dermal elements – i.e. formed in association with the skin.
Deeper elements – vertebrae, fin skeleton, fin rays – are still cartilage. In sharks,
the plates are lost and the scales turn to denticles.
Skeleton: in bony fish, the head armour is retained and creates the skull. In bony
fish, the vertebrae, fin skeleton, fin rays are turned into bone.
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